1. Technical Field
This invention relates to controlling the flow of current to the coil of a solenoid.
2. Background Art
Various circuitry for driving solenoids is known. For example, it is known to apply a driving current to a solenoid in accordance with a periodic function, such as a square wave, thus energizing the solenoid with an average current less than the maximum applied current. It is also known that after a solenoid is energized and initial displacement has taken place, a reduced amount of power is necessary to maintain the solenoid in an energized condition. Thus, it is possible to reduce power consumption in a solenoid by initially applying a higher peak current magnitude and then reducing the current to a lower sustaining value. Such a current reduction can take place, for example, after a certain amount of time has passed. However, reliance upon the passage of a predetermined amount of time may be undesirable in that it may not accurately reflect the actual condition and current requirements of the solenoid. That is, current may be reduced before the solenoid is fully energized or current may be maintained at a high level an unnecessarily long period of time after the solenoid is energized.
Specific examples of circuitry for driving solenoids include U.S. Pat. No. 4,180,026 to Schulzke et al which teaches a pair of transistors to drive a solenoid. One of the transistors is turned on only between driving periods. Solenoid driving circuits with two transistors are also taught in patents to Ohba, U.S. Pat. Nos. 4,347,544 and 4,360,855. U.S. Pat. No. 3,581,156 to Dolbachian et al teaches an electromagnetic clutch driver having switches by which the clutch coil can be driven in a variety of modes. U.S. Pat. No. 4,327,394 to Harper teaches a relatively slow decay from a peak voltage to a sustaining voltage. Such a slow decay can be unacceptable for proper actuation of fuel injectors. The circuit taught by Harper is constrained from speeding up the decay by the time constant due to the inductance and resistance of the circuitry.
In particular, it is known to use a switching coil driver to control current to automotive fuel injector and transmission solenoids and to use switching (on-off) techniques to both minimize power dissipation and, in some cases, minimize solenoid non-linearity and hysteresis.
A solenoid driver may supply current to the coil as a current sinking or a current sourcing device. As a current sinking device, one side of the coil is connected to the battery. The solenoid is turned on by grounding (sinking) the other side of the coil through a switch such as a transistor. As a current sourcing device, one side of the coil is connected to ground. The solenoid is turned on by connecting the other side of the coil to battery voltage through a switch. This configuration has the advantage of protecting for an accidental short to ground in the wiring harness between the driver and the solenoid. If this happens the solenoid will turn off rather than on, as would happen with the current sinking configuration. Turning the solenoid off is a preferred failure mode since it is advantageous to have the primary failure mode (open electrical connection) the same as the secondary failure mode (short to ground). Both configurations have the advantage of only requiring one wire from the driver to the solenoid.
A publication by SGS-ATES Semiconductor Corporation in June 1982 entitled "Injector Driver Control--Tentative Data Sheet" discloses a current sinking device with a series transistor controlling flow through a solenoid coil and a sensing resistor. A second transistor selectively provides a current path parallel to the solenoid coil. The two transistors are controlled to reduce solenoid current from an initial peak current to reduced magnitude sustaining currents.
Even though reducing solenoid driving current from a peak current to a sustaining current is known, it is still desired to obtain a means to further reduce power dissipation and minimize nonlinearity in solenoid output in response to an input having a duty cycle. It would be desirable to avoid such limitations. These are some of the problems this invention overcomes.